Devices for energy storage and energy transformation are known in the practice in particular as ignition coils, which represent an energy-transmitting high-voltage source and in engines operating according to the spark ignition principle, are used to activate a spark plug, which in turn ignites the fuel mixture in the combustion chamber of the internal combustion engine. In such an energy storage device and transformer embodied as an ignition coil, comparatively low supply voltage electrical energy, normally from a direct current vehicle electrical system, is converted into high-voltage electrical energy at a desired point in time at which an ignition pulse is to be delivered to the spark plug.
To convert electrical energy into magnetic energy, the system current of the motor vehicle flows through a first coil, which is customarily a copper wire winding, as a result of which a magnetic field forms around this coil, the magnetic field having a specific direction and being a closed-line magnetic field. To deliver the stored electrical energy in the form of high-voltage pulses, the previously built-up magnetic field is forced to change its direction by cutting off the electric current, causing an electrical high voltage to be formed in a second coil, which is located physically close to the first coil and has a much higher number of turns. The conversion of the now electrical energy at the spark plug causes the previously built-up magnetic field to break down and the ignition coil to discharge. The design of the second winding makes it possible to set high voltage, spark current and spark duration in the ignition of the internal combustion engine as needed.
All ignition coils have an I core made of a ferromagnetic material such as iron, for example. The I core is thus a rod-shaped or rectangular iron core, the cross-section of which may be made up of lamellae of soft iron sheet. In the known related art, the placement of the coils and of the I core is subject to great variation; however, the coils are usually superposed radially and are positioned concentrically to the I core. It is also customary in practice to provide, in addition to an I core of this type, a peripheral core made of ferromagnetic material, which surrounds the longitudinal extent of the coils and is also described as an “O core” or “ferromagnetic circuit.” In order to reduce losses when building up and breaking down the magnetic field, this peripheral core is also normally a combination of layered iron lamellae.
In order to be able to install the windings or coils, the I core and the peripheral core of a ferromagnetic circuit may not be of one piece but instead must be assembled from different component parts. A typical configuration is the construction of an I core and an O core forming a closed O, the I core together with the windings surrounding it being inserted into the interior of the O core at the time the ignition coil is assembled so that the lamella stacks of the cores lie in one plane when installed.
In order to influence the magnetic field in a specific way, the ferromagnetic circuit is normally interrupted by spaces or air gaps, this being referred to as a “magnetic shear.” A permanent magnet may also be located in such a space, making a further increase in the magnetic energy possible under specific conditions. The system of such air gaps and permanent magnets is preferably located at the joints between the I core and the O core.
A problem with the known devices for energy storage and energy transformation designed as ignition coils is that assembly gaps which are based on the manufacturing tolerances and the insertion play for inserting the I core into the O core must be maintained in the design of the magnetically active core elements. These gaps may be incompatible with the gap dimensions desired based on energy considerations.
Thus, for example, when a permanent magnet is positioned at one end area of the I core between the I core and the O core, no air gap is desired between the permanent magnet and the O core. The air gap that must be provided for manufacturing reasons must be compensated by appropriate measures or derivative actions, which are reflected in the overall dimensions and ultimately in additional costs as well.
U.S. Pat. No. 7,212,092 to Bosch discloses a device for energy storage and transformation that overcomes some of the problems addressed above. Referring to FIG. 1, a compact ignition coil has a centrally positioned magnetically soft I-core. A first coil former 2 is positioned concentrically surrounding the magnetically active I core, a winding connected to a supply voltage from a vehicle electrical system and used as a primary winding being applied to coil former 2. Situated radially within the first coil former 2 is a second internal coil former 3, which surrounds the I core and has a winding used as a secondary winding connected to a high-voltage terminal connected to a spark plug. In an end area, the I core 1 is situated within coil formers 2 and 3 and has a permanent magnet 4. The I core, with coil formers 2 and 3, is inserted into a through recess in peripheral core 5. An assembly gap 6 that compensates for manufacturing tolerances is situated between permanent magnet 4 and peripheral core 5. The gap 6 may be closed by the force of permanent magnet 4 in various embodiments. In this device, the permanent magnet is accommodated between two separate parts of the magnetic core. In this configuration, it is possible to achieve higher energy from the coil due to the non-linearity of the primary current versus time only when the magnetic area is realized on the I core with zero gaps at all interfaces between the primary and secondary coils.